The present invention relates to a a method for manufacturing a glass substrate for an information recording medium.
A conventional glass substrate has a surface strength improved by the following chemical reinforcing treatment. The glass substrate is immersed in a chemical reinforcement liquid, or molten potassium nitrate (KNO3). In the glass substrate, metal ions having a relatively small ionic radius, such as lithium ions (Li+) and sodium ions (Na+), are present. The chemical reinforcing treatment causes the metal ions having a relatively small ionic radius present near the surface of the glass substrate to be replaced by potassium ions (K+) having a relatively large ionic radius contained in the chemical reinforcement liquid to form a compression stress layer on the surface of the glass substrate, so that the glass substrate is reinforced chemically.
The chemically reinforced glass substrate is polished using a diamond slurry and an abrasive tape. This polishing forms a texture in the surface of the glass substrate.
Potassium ions incorporated into the glass substrate by the chemical reinforcing treatment are unlikely to ooze from the surface of the glass substrate. However, most of the compression stress layer in which potassium ions are present in a higher concentration is removed during formation of the texture, so that a layer containing sodium ions and lithium ions in a relatively high concentration on the surface of the glass substrate is exposed. The sodium ions and lithium ions having a relatively small ionic radius move relatively easily in the glass substrate. Therefore, an alkali metal, such as sodium or lithium, oozes from the glass substrate.
In the glass substrate from which an alkali oozes, a texture formed in the surface of the glass substrate changes with time. In a magnetic recording medium manufactured using a glass substrate having a texture which deformes to an undesirable form, the recorded magnetic data cannot be stably read by a magnetic head with high accuracy.